Filtration is often employed in water treatment as well as other chemical processes as a means for separation of phases, specifically solids from liquids or gases. A common filtration process is that of surface filtration which, because of pressure drop considerations, usually requires a large filtration surface area. To effect a large filtration surface area in a vessel many filter elements or filter septa can be used which are attached to a tube sheet.
Further application of the surface filtration process has been extended to ion exchange and filtration simultaneously by first coating the filter element with a filtration medium and/or powdered ion exchange resins. This application increases the performance in that some degree of depth filtration can be utilized in addition to surface filtration. Demineralization can also be accomplished simultaneously.
The materials applied before in this case are referred to "precoat" materials as they are applied before the system is put into actual service. If they are applied during the service cycle, they are referred to as "body feed" materials.
Surface filters, precoated filters and precoated and body fed filters have been used extensively in the water treatment and the chemical process industries. To date, there are many filter septa to tube sheet attachment mechanisms which are being employed. Since rather large filter vessels with as many as several hundreds of septa are used, there is frequently leakage through the attachment mechanism, thus allowing passage of upstream components of the process stream into the filtrate. The primary reason that the attachment mechanism fails is that it is often quite complex and requires remote handling tools. Commonly, if the filter septa has substantial strength, the septa is used as the remote tool device.
For long filter elements, on the order of 5 to 7 feet in length, complex spring attachment mechanisms, twist lock mechanisms, and simple threaded mechanisms are common. Again, during the repeated service and cleaning cycles these types of mechanisms are prone to be loosen and lead to subsequent service cycle leakage. Since usually a seal or gasket is required, any rotating action required during element insertion tends to deform the gasket material which also leads to eventual leakage while in service.
In many instances, any leakage of particulates or ions through the system can have very detrimental effects on the overall process operation and on materials in contact with the filtrate. For example, in boiler feed water treatment as found in, for example, steam generation water systems, ion exchange filtration is used to purify the water so that high alloy steels in the boiler do not fail. If ion exchange resins leak past the water treatment system, excessive alloy steel corrosion is effected rapidly, often requiring plant shut down and costly repairs.